boot: use BUILD_ID if VERSION_ID is not present

According to os-release(5), VERSION_ID is not mandatory and BUILD_ID only
needs to be unique underneath VERSION_ID. Therefore, assuming a missing
VERSION_ID field means 'empty', we can rely on BUILD_ID to be unique.

Use BUILD_ID if VERSION_ID is not present. This way, rolling-release
distros can still provide a proper os-release entry without crafting
random VERSION_ID strings.

This fixes #186.